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Abstract

We directly visualize and identify the capacitive coupling of infrared dimer antennas in the near field by employing scattering-type scanning near-field optical microscopy (s-SNOM). The coupling is identified by (i) resolving the strongly enhanced nano-localized near fields in the antenna gap and by (ii) tracing the red shift of the dimer resonance when compared to the resonance of the single antenna constituents. Furthermore, by modifying the illumination geometry we break the symmetry, providing a means to excite both the bonding and the “dark” anti-bonding modes. By spectrally matching both modes, their interference yields an enhancement or suppression of the near fields at specific locations, which could be useful in nanoscale coherent control applications.

Figures (7)

Fig. 1 Experimental set-up and near-field imaging. (a) Illustration of the s-SNOM used for mapping the near-field distribution and topography of infrared dimers. The Si tip, which vibrates at the mechanical resonance frequency Ω of the AFM cantilever, is used to scatter the antenna fields. Using a parabolic mirror objective, the dimer is illuminated with the focused beam of a CO2 laser (Einc), which is polarized parallel to the long axis of the antennas (s-polarization). The same objective is used to collect the backscattered light (Eff). A polarizer in front of the detector ensures the selection of either s-polarized or p-polarized backscattered fields. Signal demodulation at higher harmonics nΩ in combination with a pseudo-heterodyne interferometric detection yields background-free near-field amplitude |En| and phase φn maps [50]. (b) Topography and near-field images of a dimer antenna for (c) p-polarization (|E4|p, φ4p) and (d) s-polarization (|E4|s, φ4s) detection schemes. The imaging wavelength is λinc = 11.1 μm. The dashed white line in the phase images highlight the nanorods contour.

Fig. 2 Diagram of energy levels as a function of nanorod length L for dimer antennas and single nanorods. At a fixed illumination wavelength λinc (red dashed line) the lower energy bonding mode in a dimer antenna (red) occurs at a shorter nanorod length L1 than the fundamental.

Fig. 4 Verification of near-field coupling in dimer antennas. (a) Near-field amplitude |E4|s and (b) phase φ4s images of dimer antennas with a varying length L. The horizontal white lines separate the images taken individually. (c) Comparison of the normalized near-field amplitudeE4sin dimer antennas (red dots) and single nanorods (blue dots) as a function of nanorod length L. (d) Comparison of the near-field phase φ4s/2 in dimer antennas (red dots) and single nanorods (blue dots, data from ref [49].) as a function of nanorod length L. The crosses in the antenna schematics show the locations were the fields were evaluated: the center of the gap for the dimers and the nanorod extremity for the single nanorods. Numerical calculations by FDTD of the in-plane component of the antennas’s near-field amplitude and phase are also shown in (c) and (d) by red (dimers) and blue (single nanorods) solid lines.

Fig. 7 Interference of modes in rotated dimer antennas. (a) Experimental (|E3|p, φ3p) and calculated out-of-plane near-field amplitude and phase images for a nanorod length LA = 2.7 μm and (b) a nanorod length LB = 3.8 μm. (c) Numerically calculated values of the near-field amplitude and (d) phase of the out-of-plane near-field component at the left/right side of the gap (red/blue curve). The red and blue crosses mark the positions at 300 nm from the gap on top of the antennas where the near-field amplitude and phase were extracted from the near-field images. For comparison the near-field amplitude and phase for non-rotated antennas (grey curves) evaluated at the same location are also shown.